optimizing the stacking sequence in dual-purpose body armours

Optimizing the Stacking Sequence in Dual-Purpose Body Armours

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Ian Horsfall, Celia H Watson, Steve M Champion

Department of Engineering and Applied Science, Cranfield University, Shrivenham,

Wiltshire. SN6 8LA, United Kingdom

Tel: +44 1793 785389, email: [email protected]

Abstract - Paper for the 27th ISB

Background. Many police body armour systems are dual purpose, offering both ballistic and

knife resistance by combining a flexible ballistic textile pack with a stiffer knife resistant

layer. The two types of protection differ in materials and mechanisms such that each

individual component may help or interfere with the function of the other. This paper

investigates the effect on knife and ballistic penetration resistance when a single thin metal

plate was placed at various different positions within an aramid textile armour pack. Two

metallic layers were used, aluminium 7075 and commercial purity titanium, these had similar

areal densities and were positioned in the front, middle and back of a 20 layer pack of woven

Kevlar® 49.

Method of Approach. An instrumented drop weight machine was used to deliver a

repeatable knife blade impact at comparable energy levels to those specified in UK Home

Office test standards for knife resistance. Ballistic tests were used to determine the V50

ballistic limit velocity against typical 9mm and .357” Magnum handgun threats.

Results. Against a stabbing threat, it was found that positioning the metal plate in the middle

of the pack provided the greatest resistance to knife penetration by a factor of almost two.

Whilst a plate at the front of the pack provided less resistance and plates positioned at the rear

of the pack provided the least resistance to penetration. Against the ballistic threat, the

penetration resistance of the textile pack can be significantly improved when a metal plate is

at the front of the pack whilst for all other positions the effect is negligible. However, this

effect is sensitive to both the ammunition type and the metal plate composition. When the

metal plate is positioned at the rear of the pack there is a significant decrease in the back-face

deformation of the armour pack although again this effect is only present for certain

ammunition and metal combinations.

Conclusions. The overall effect of combining soft and hard elements was that specific

performance parameters could be substantially increased by the correct combination. There

were no significant negative effects, but in a number of cases the combined systems

performance was no greater than that of a single element type, despite the added weight.

Keywords. Body armour, ballistic protection, stab resistance, textile armour.


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1. Introduction

To maximise ballistic resistance, textile armour packs need to be strong enough to resist

perforation and flexible enough deform and exchange kinetic energy with the bullet [1-3].

Body armours made of layers of woven aramid textiles have been developed so that as the

projectile impacts and begins to perforate the layers of aramid, each of the layers are strong

enough to deform the projectile before perforation [4]. This mechanism has the effect of

increasing the presented area of the projectile, which makes the perforation of each

successive layer more difficult as the contact stress is reduced [5]. The armour panel is also

forced backwards and driving this motion absorbs some of the impact energy until all of the

impact energy has been dissipated [6].

A bullet such as the .357” Magnum has an initial cross-section of 65mm2 and after impact

can mushroom to a cross-section of up to 254mm2. The result is that optimized textile

ballistic armour tends to consist of a flexible system, which operates by deforming rearwards

when struck. In contrast the tip of a knife will have a very small cross-sectional area ranging

from as little as 0.2mm2

for a sharp tip to 2.5mm2

for a blunt knife [7]. Maximising knife

resistance in body armour requires a strong stiff panel, as the energy concentration over such

small cross-sectional areas, mean that the energy per unit area at the tip of a knife will be

high. Therefore knife armour needs to be relatively hard and typically inflexible in order to

prevent immediate penetration. Flexibility can be introduced into knife resistant armour by

incorporating a layer of chain mail. Although chain mail is effective, it consists of metal rings

which may result in differing performance depending on whether the blade tip strikes in the

centre or at the side of a ring. This will inevitably introduce some variability to the

perforation results. Multiple layers of woven aramid with coatings are also used to make stiff

knife resistant layers in body armour [8-10].

In the present work, monolithic metallic plates were chosen as the knife resistant element in

order to provide a repeatable and relatively simple solution, which is still representative of

some current designs. In addition, the metal plates mimic the effect of rigid equipment or

accessories that might be attached to the armour and are in contact with the ballistic armour

pack.

2. Materials

The knife resistant layers were; 400mm square, aluminium alloy 7075-T6 (1.5mm thick) and

titanium CP Grade 3 (1.0 mm thick) these have comparable areal densities of 4.48kg/m2 and

4.51kg/m2 respectively. Their mechanical properties were measured and are shown in table 1.

Aramid ballistic packs with an areal density of 3.53 kg/m2

were constructed from twenty

400mm squares of high tenacity, high cut resistance Du Pont Kevlar® 49, woven at 7x7

yarns per centimetre and box quilted on a 30mm pitch [11]. This enabled a completed

metal/Kevlar® composite pack with either of the metallic layers to have an areal density of

approximately 8 kg/m2 and total thickness of approximately 7mm.


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Table 1. Material property data for Aluminium and Titanium

Metal Type Density

(kgm-3

)

Yield

strength

(MPa)

Tensile

strength

(MPa)

Elongation

to fail (%)

Hardness

(Hv)

Aluminium

Alloy 7075 T6

Titanium CP

Grade 3

Kevlar® 49

2800

4510

1.45

511

275

581

450

3000

11

32

2.4

145

201

3. Ballistic tests

V50 ballistic limit tests were used to evaluate the armour systems. A V50 is the velocity at

which 50% of the shots fired are stopped by the armour and 50% perforate the armour.

Normally this value is obtained from firing six shots within a specified range of velocities.

The range (spread) of velocities allowed for a six shot V50 typically needs to be less that

40ms-1

between the lowest recorded velocity for a perforation and the highest velocity

recorded for a stop. [12]

The ammunitions, range setup, procedure and the conditioning of the witness block outlined

in the Home Office Scientific and Development Branch’s 2007 ballistic standard [13] were

followed. Details of the ammunitions are given in table 2. The 9mm ammunition has a full

metal jacket, which encloses the nose of the projectile whilst the .357” Magnum ammunition

has a jacket only around its base with an exposed lead core at the nose. Consequently, the

.357” Magnum bullet tends to mushroom rapidly even against soft targets whilst the 9mm

bullet tends to deform much less easily. All targets were mounted onto a conditioned block

of Roma Plastilina® No1. When correctly calibrated this material is recognised as an

international standard method of recording the backface deformation caused by ballistic

impacts on body armour systems [13-15]. The mean backface deformation caused by the

three non-penetrating shots (from 6 shot V50 set), was recorded.

Table 2. Ammunition used in ballistic tests

Ammunition Description Manufacturer Mass(grams)

9x19mm Parabellum

Full Metal Jacket

Dynamit Nobel

DM11A1B2

8.0g

(124 grain)

.357” Magnum

Semi-Jacketed

Soft Point Flat Nose

Norma 19107

10.2g

(158 grain)


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4. Knife tests

For the knife tests a Rosand accelerated weight instrumented impact tester type IFW8 was

used to deliver repeatable and accurate impacts of known energy. The forces resisting the

blade during the test were measured using a calibrated load cell [16], and the total drop mass

was 2.3kg. The test used the No 5 blade from UK Home Office 1993 standard [17] which is a

robust ‘Bowie’ type blade, Fig. 1.

Figure 1. Knife test blade, the UK Home Office number 5 design [17]

Test samples were clamped underneath the drop tower and supported by a Roma Plastilina®

No.1 filled tube. The penetration resistance of the 20 layer aramid pack and both metal

panels were tested for knife resistance separately, following this further test were conducted

with the metal panels were placed either at the front, back or in the middle of the aramid

packs. After each test the knife was withdrawn and the Plastilina® block, Fig. 2, was

sectioned along the axis of the penetration, exposing the penetration witness mark which

allowed the depth of penetration to be measured, [18-20].

Figure 2. Sectioning the Plastlina® test block to measure depth of penetration

5. Results - Knife resistance

The individual components of the test pack were tested in the drop tower for their knife

resistance using a range of energies from 4 to 54joules, and compared by plotting the knife

penetration (mm) against impact kinetic energy (joules) as shown in Fig. 3.


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Figure 3. Comparison of the penetration resistance of the individual armour components

As expected the 20 layer aramid pack had the least resistance to penetration with the aramid

strands being cut cleanly and a maximum penetration of 44 mm being reached at 26joules.

Aluminium showed the greatest resistance to penetration with 26mm of penetration for a 43

joule impact, in comparison to 36mm for a 44joule impact on titanium. The force

displacement curves from typical tests are shown in Fig. 4. The aluminium produces a steep

rise in force to a peak at only 3mm of displacement after which perforation occurs. In

contrast, the titanium and aramid packs show a more gradual increase in force up to

perforation at approximately 10mm displacement and less subsequent drop in force. This is a

result of the relatively high stiffness of the aluminium plate leading to a sudden failure whilst

the other systems deformed into the Plastilina® block and fail in a less sudden manner.


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Figure 4. Force vs. displacement graphs of individual components,

20 layer Kevlar® aramid pack, aluminium and titanium sheets.

Aluminium and titanium sheets were then positioned in front of, behind and in the middle of

the aramid packs. These composite packs were tested at energies from 40 to 60joules, with 5

drops carried out at each energy level, Fig. 5. For both metals the system gave the greatest

knife resistance when placed in the middle of the pack. For 60joule impacts positioning plates

in the middle produced half the depth of penetration of front positioned plates and one third

the depth of penetration of back positioned plates.


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Figure 5. Effect of position of metal plates on knife resistance

Figure 6 shows the force vs. displacement graphs for aluminium plates at the three positions.

The curves are seen to be more complex than for the plates in isolation, Fig. 4. For the test

shown in figure 6 perforation of the aluminium plate is characterized by a sudden drop in

force and subsequent rise, this is seen at 8mm displacement for the plate in the front position

and between 16mm and 18mm displacement for the other two positions. It should also be

noted that the load at peak is significantly higher when the plate is in the front position.

Figure 6. Effect of aluminium plate position on knife penetration force


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Figure 7. Perforation of aluminium plate showing failure, a) Aluminium at front of aramid

showing cut edge and brittle crack, b) Aluminium in middle of aramid, c) Aluminium at back

of aramid

Examination of the perforated plates illustrated in Fig. 7 shows a distinct difference in failure

mechanism between the in front position and the other two. When positioned at the front the

failure is seen to be brittle with a row of chips being present along the rear face of the fracture

(figure 7a). For the other two positions, there is a clear shear surface visible at approximately

450 to the plane of the plate. The failure mode in this case appears to be the result of

membrane stresses causing a relatively ductile, tensile induced shear failure in the plate.. In

all three cases there is relatively little deformation other than local deformation of the

material in contact with the blade.

For titanium there is less apparent difference in the failure mode with all three plates showing

local dishing around the impact site and a relatively ductile failure of the plate. Figure 8

shows the force displacement curves for the titanium samples and Fig. 9 shows the perforated

plates.

Figure 8. Effect of Titanium plate position on knife penetration force

a b c


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Figure 9. Perforation of titanium plate showing failure, a) Titanium at front of aramid

showing cut edge, b) Titanium in middle of aramid, c) Titanium at back of aramid

6. Results - Ballistic resistance

To determine the ballistic limit, tests were carried out against 9mm and .357” Magnum

ammunitions on the 20 layer aramid packs only. Then the same tests were performed with

the metal plates placed in front, middle or at the back of the 20 layer aramid packs. For each

construction theV50 ballistic limit velocity was determined using the procedure of STANAG

2920 [12],using 6 shots; 3 perforating and 3 non-perforating. The sample standard deviation

(SD) of the individual shot velocities was calculated in order to provide a measure of the

spread of data. The backface deformations were measured from the 3 non-penetrating shots

and the SD of these was also calculated (except for 9mm vs. titanium front where insufficient

data was available to provide an estimate of SD).

Table 3. V50 Ballistic limit tests on aramid packs and aluminium and titanium plates with

aramid packs

Against the 9mm bullet, the aluminium panel tended to petal with large cracks extending up

to 25mm away from the impact site. Compared with the results obtained without a metal

plate, placing the plate in front of the aramid significantly increased the armours’ ballistic

resistance against 9mm bullets (table 3). The metallic plate at the front was effective at

deforming the 9mm bullet and reducing the initial impact energy of the bullet before impact

Ammunition Armour V50

(ms-1

)

Velocity

spread

(ms-1

)

SD

(ms-1

)

Backface

deformation

(mm)

SD

(mm)

9x19mm Para

.357” Magnum

Aramid only

Aramid only

339

445

25

33

8.4

11.9

49

56

2.1

4.0

9x19mm Para Aluminium front 432 27 9.8 43 9.3

Aluminium middle 357 11 4.6 56 3.2

Aluminium back 339 24 11.9 32 3.5

.357” Magnum Aluminium front

Aluminium middle

Aluminium back

468

419

454

13

32

13

5.4

11.0

5.2

54

69

48

10.7

14.2

7.1

9x19mm Para Titanium front 435 20 8.7 46 NA

Titanium middle 417 26 9.9 58 4.2

Titanium back 368 9.0 3.7 24 6.0

.357” Magnum Titanium front

Titanium middle

Titanium back

426

440

458

8.0

28

5.0

3.2

10.9

2.0

52

62

19

8.5

8.7

2.9

a b c


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on the aramid pack. This resulted in a 92ms-1

increase in ballistic performance (339ms-1

to

431ms-1

) with a 10mm reduction in backface deformation. The ballistic resistance against the

.357” Magnum increased by only 23ms-1

(445ms-1

to 468ms-1

) and very slight (2mm)

reduction in backface deformation. In both cases the change in backface deformation was not

statistically significant.

Placing aluminium plates in the middle of the pack produced only slight changes in

performance compared to the aramid alone and did not show the same increased performance

of the front positioned plates. Also, the greatest backface deformations were measured when

the plate was in the middle position.

The titanium results with the 9mm ammunition follow the same trends as those of the

aluminium, with the front position giving the best ballistic resistance and the rear position

giving the lowest backface deformations. For the 0.357” Magnum, the effect of the titanium

plate was negligible for all locations with ballistic resistance and backface deformation

similar to the aramid by itself. The only exception being that the titanium plate at the back

reduced the backface deformation by a factor of more than two.

7. Discussion

The energy delivered by the knife is absorbed by two mechanisms; direct resistance to

penetration from the armour plate and backwards movement of the armour by dishing.

Backwards movement of the armour also reduces the contact load allowing a given armour

system to have an apparently better penetration resistance. The effect of introducing a

relatively stiff metallic plate into an armour system is to restrict the backward movement of

any elements placed in front of this layer. Therefore, it could be assumed that placing a

metallic layer in front of an aramid pack would produce the best knife resistance, with middle

or rear positioned metal layers being increasingly worse. The results for aluminium in Figure

5 showed that this was not the case, where aluminium plates in the middle gave the best

performance and aluminium plates at the rear had the lowest performance.

The presence of even part of the aramid pack in front of the aluminium changes its failure

mode from a brittle cutting failure to a ductile bursting failure. Although the cutting failure

produces an initially higher resistive loads (fig. 6) the overall performance of the system is

not as good as that achieved for the middle mounted plates (fig.5). Possibly the ability of the

mid mounted plate to move backwards into the rear aramid layer reduces the contact load and

allows more time and distance over which the perforation of the blade can be resisted.

Titanium panel armour had similar force profiles regardless of position with the only effect

being a slight change in the initial force gradient. The titanium panel is relatively more

flexible than the aluminium panel and stays in contact with the knife for longer during the

event resulting in similar values for the peak loads for all three plate positions

This suggests that the dominant mechanism in the initial contact stage is the ability of the

plate to move or deform rearwards. Whilst after perforation the dominant effect is frictional

resistance between the knife and the metal plate. Previous work [7] has suggested that both

these effects are present and provide the main components of knife resistance.


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Placing both types of metal plate in front of the aramid in the ballistic tests, allowed the plates

to deform the bullet and absorb some energy before contact with the aramid pack. The bullet

impact on the plate may also cause some acceleration of the aramid away from the bullet

prior to direct contact reducing its relative velocity. These mechanisms improved the ballistic

limit velocity but the backface deformations were unchanged compared to the aramid packs

alone.

Placing the metal plates at the rear of the armour system restricted the movement of the

aramid which reduced the ballistic limit. However, the packs deformed the projectile,

absorbed energy and reduced the velocity before the projectile struck the metal layer. The

plate then absorbed more energy from the projectile, which reduced the amount of backface

deformation. Restricting movement reduces the distance and time the system has to stop the

projectile and produce high contact stresses. These high contact stresses increased the

deformation of both 9mm and .357” Magnum projectiles as the metal plates were moved

from the front of the system, to the middle and the rear. The titanium plate was more ductile

than the aluminium and had greater reductions in backface deformation. This was due to the

titanium deforming in concert with the aramid and dissipating the impact loads over the

surface of the witness block. Whereas the aluminium fractured and peeled back allowing the

aramid to be pulled through the resulting hole. The most significant reduction in backface

deformation was achieved by placing the titanium to the rear of the panel

Metal plates situated in the middle of the armour system resulted in relatively poor ballistic

limit performance and greater backface deformation results. The ten layers of aramid in front

of the metal were not sufficient to cushion the effects of the relatively stiff metal plate and

prevented the aramid from moving and performing at its best. The remaining ten layers of

aramid behind the plate is not enough to maintain the ballistic limit or stiff enough to control

the backface deformation. For the 0.357” Magnum tests placing either plate in the middle

position actually reduced ballistic limit velocity but the reduction was relatively small and

was not statistically significant.

8. Conclusions

Placement of anti-stab systems within a dual purpose armour system is of importance, and

can be used as a method of tuning the properties of that system to a particular requirement.

In complex stab resistant armour systems perforation resistance is a result of both the intrinsic

resistance and mechanical interaction of the armour elements. In systems containing rigid and

penetration resistant elements it is preferable to provide both forward and rearward

cushioning so that stresses are transmitted ahead of the penetrating knife and rearwards

movement is allowed in the direction of penetration. For a simple dual purpose armour

system the optimum configuration is probably one in which the metallic knife resistant

element is in the centre of the textile ballistic packs. This achieves good knife performance

with only limited reduction in ballistic performance.

If a metal plate is placed on the front of a aramid armour pack to gain anti-stab performance,

it is preferential for ballistic performance to make this layer as hard as possible. If however a

metal plate is to be placed elsewhere in the armour system then a more ductile material is

preferable.


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This work suggests that care should be taken if hard items or layers are positioned

immediately behind a textile armour. Although no significant reduction in performance was

observed in this work it should not be assumed that an extra layer will increase ballistic

resistance.

9. Acknowledgements

The authors would like to acknowledge the efforts of A.M. Robertson in the completion of

this research.

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optimizing the stacking sequence in dual-purpose body armours

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